Meanwhile, combine the butter, sour cream, eggs, orange zest, ginger, 2½ teaspoons salt, and 1 teaspoon pepper in a medium bowl. Whisk in the hot purée, pour it into a 9 x 12 x 1½-inch oval gratin dish, and smooth the top. Bake for 40 to 50 minutes, until slightly puffed and golden on top. Serve hot.